[JMLot]

The concept of unions themselves revolve around the idea of people having more bargaining power when they join together but now you're effectively saying to ignore the strength of certain unions and treat them all equally.

Why bother with unions in the first place?

I'd be ok with this part, I am not pro-union anyway. But to answer your question it is about collective bargaining, specifically removing it from one union but allowing it in others. Granted the size of the union in question and it's strength (and importance as teachers) gives them great power but as I understand it this legislation is being introduced because they will not give the concessions the Republicans want. The issue at hand is not the strength of one union over another, it is about a level collective bargaining right for them all to have. In the end you have a discrimination legislation effort to get around only this unions right to collective bargaining over others. If for instance teachers unions being larger have gotten them better contracts then the issue becomes how to negotiate with them, rather how to demand concessions in the face of budget crisis. But, in this case Republicans in the state have opted to try a biased legislation effort, one that will be a mess to sort out if it passes.

Discrimination would imply someone is being denied a legal right but I don't see any legal rights to ensure everyone gets the same contract.

No, in this case it is not about the same contract at all. It is not about the right for all of them to have similar contracts. It is about the right to pursue contracts for one's union specifically. As I said above, it is about removal of collective bargaining for only some unions but allowing them for others. That is discriminatory and will certainly be challenged.